Method and apparatus for processing digital image to be displayed on display device with backlight module

Abstract

A method and apparatus for processing a digital image to be displayed on a display panel illuminated with a backlight module. The backlight module is set to a specific backlight duty according to a luminance statistic of the digital image. The digital image is remapped through a tone mapping function formed according to the specific backlight duty and the luminance statistic.

1. A method for processing a digital image which is to be displayed on a display panel illuminated with a backlight module, the method comprising the steps of:
 generating a luminance statistic of said digital image; 
 determining a specific backlight duty according to said luminance statistic; 
 setting said backlight module to said specific backlight duty; 
 forming a tone mapping function according to said specific backlight duty and said luminance statistic; and 
 remapping said digital image by said tone mapping function before said digital image is displayed on said display panel; 
 wherein luminance value of each pixel in said digital image is encoded with a predetermined bit resolution and said forming of a tone mapping function comprises:
 determining a linear upper limit mapping function having a specific slope R 1  determined according to said specific backlight duty, said linear upper limit mapping function remapping code value zero to zero; 
 determining a first luminance value X 1  according to said specific slope R 1 ; 
 determining a second luminance value X 2  according to said first luminance value X 1  and the maximum code value Cmax in said predetermined bit resolution; 
 determining a lower limit factor R 2  greater than or equal to one, and less than said specific slope R 1 ; and 
 forming said tone mapping function such that said second luminance value X 2  is remapped to a specific remapped value (W*R 1 +(1−W)*R 2 )*X 2 , wherein W is a weighting number lying between 0 and 1 which is determined according to said luminance statistic, and wherein any luminance value less than said first luminance value X 1  is remapped through said linear upper limit mapping function. 
 
 
     
     
       2. The method of  claim 1 , wherein said tone mapping function is further characterized in that said maximum code value Cmax is remapped to itself. 
     
     
       3. The method of  claim 2 , wherein said tone mapping function is further characterized in that any luminance value lying between said first luminance value X 1  and said second luminance value X 2  is remapped through a first linear mapping function defined by a straight line connecting points (X 1 ,R 1 *X 1 ) and (X 2 , (W*R 1 +(1−W)*R 2 )*X 2 ), and any pixel with a luminance value greater than said second luminance value X 2  is remapped through a second linear mapping function defined by another straight line connecting points (X 2 , (W*R 1 +(1−W)*R 2 )*X 2 ) and (Cmax, Cmax). 
     
     
       4. The method of  claim 1 , wherein said first luminance value X 1  is determined such that said first luminance value X 1  is remapped to a value less than said maximum code value Cmax through said linear upper limit mapping function. 
     
     
       5. The method of  claim 1 , wherein said lower limit factor R 2  is equal to one. 
     
     
       6. The method of  claim 1 , wherein said weighting number W substantially equals a ratio of count of pixels with luminance values lying between said first luminance value X 1  and said second luminance value X 2  to count of pixels with luminance values lying between said first luminance value X 1  and said maximum code value Cmax in said digital image. 
     
     
       7. The method of  claim 1 , wherein said first luminance value X 1  and said second luminance value X 2  are Cmax/2 and (Cmax+X 1 )/2 respectively. 
     
     
       8. The method of  claim 1 , wherein said luminance statistic is a histogram recording occurrence frequencies of all possible luminance values of pixels in said digital image. 
     
     
       9. The method of  claim 1 , wherein said display panel is a liquid crystal display panel. 
     
     
       10. An apparatus for processing a digital image which is to be displayed on a display panel illuminated with a backlight module, comprising:
 an image analysis unit, configured for generating a luminance statistic of said digital image, and determining a specific backlight duty according to said luminance statistic; 
 a backlight setting unit, capable of setting said backlight module to said specific backlight duty; 
 a tone mapping function generator, configured for forming a tone mapping function according to said specific backlight duty and said luminance statistic; and 
 a tone remapping unit, configured for remapping said digital image by said tone mapping function before said digital image is displayed on said display panel; 
 wherein luminance value of each pixel in said digital image is encoded with a predetermined bit resolution, and wherein said tone mapping function generator comprises:
 means for determining a linear upper limit mapping function having a specific slope R 1  determined according to said specific backlight duty, said linear upper limit mapping function remapping code value zero to zero; 
 means for determining a first luminance value X 1  according to said specific slope R 1 ; 
 means for determining a second luminance value X 2  according to said first luminance value X 1  and the maximum code value Cmax in said predetermined bit resolution; and 
 means for determining a lower limit factor R 2  greater than or equal to one, and less than said specific slope R 1 ; 
 wherein said means collectively form said tone mapping function such that said second luminance value X 2  is remapped to a specific remapped value (W*R 1 +(1−W)*R 2 )*X 2 , wherein W is a weighting number lying between 0 and 1 which is determined according to said luminance statistic, and wherein any luminance value less than said first luminance value X 1  is remapped through said linear upper limit mapping function. 
 
 
     
     
       11. The method of  claim 10 , wherein said tone mapping function is further characterized in that said maximum code value Cmax is remapped to itself. 
     
     
       12. The method of  claim 11 , wherein said tone mapping function is further characterized in that any luminance value lying between said first luminance value X 1  and said second luminance value X 2  is remapped through a first linear mapping function defined by a straight line connecting points (X 1 ,R 1 *X 1 ) and (X 2 , (W*R 1 +(1−W)*R 2 )*X 2 ), and any pixel with a luminance value greater than said second luminance value X 2  is remapped through a second linear mapping function defined by another straight line connecting points (X 2 , (W*R 1 +(1−W)*R 2 )*X 2 ) and (Cmax, Cmax). 
     
     
       13. The method of  claim 10 , wherein said first luminance value X 1  is determined such that said first luminance value X 1  is remapped to a value less than said maximum code value Cmax through said linear upper limit mapping function. 
     
     
       14. The method of  claim 10 , wherein said lower limit factor R 2  is equal to one. 
     
     
       15. The method of  claim 10 , wherein said weighting number W substantially equals a ratio of count of pixels with luminance values lying between said first luminance value X 1  and said second luminance value X 2  to count of pixels with luminance values lying between said first luminance value X 1  and said maximum code value Cmax in said digital image. 
     
     
       16. The method of  claim 10 , wherein said first luminance value X 1  and said second luminance value X 2  are Cmax/2 and (Cmax+X 1 )/2 respectively. 
     
     
       17. The method of  claim 10 , wherein said luminance statistic is a histogram recording occurrence frequencies of all possible luminance values of pixels in said digital image. 
     
     
       18. The method of  claim 10 , wherein said display panel is a liquid crystal display panel.